O. ramosa is incredibly common, certainly on Crete where it can form huge drifts as it is happy to parasitise Oxalis pes-caprae which is almost everywhere. The other species which I encountered all over Crete and is widespread throughout the Med is O. pubescens. I was trying to find out whether it is an annual or a perennial but have failed so far. It seems that O. hederae is an annual which is quite common here in London.
O. pubescens is very variable, at least in colour. Most on Crete were brown or very pale though I did see at least one decent purple form. Yellowish hues are also around and combinations too. This specimen in the Vikos gorge, N Greece was quite a nice purple colour. I'm reasonably sure I have identified it correctly. The other reason this species is so widespread is that it parasitises a wide range of hosts in the Apiaceae (Umbelliferaea) and Asteraceae (Compositae) from sea level to at least 2000 metres.